This is not related to XWiki 
It’s related to Tomcat. XWiki is independent of the Servlet container.
If you don’t know how to set the memory on Tomcat you should check Tomcat’s doc. We have some hints on https://www.xwiki.org/xwiki/bin/view/Documentation/AdminGuide/Installation/InstallationWAR/InstallationTomcat/#HOutOfMemoryError but it won’t replace proper Tomcat doc.